1. Field of the Invention
The present invention relates to a wire bonding method for connecting two bonding points.
2. Prior Art
The manufacturing processes of assembled semiconductor devices, such as IC's, etc., include a wire bonding process. In this process, as shown in FIG. 2, a wire 3 is connected to pads (first bonding points) 1a on a pellet 1 and leads (second bonding points) 2a on a lead frame 2. The arrows A in FIG. 2 indicate the direction of looping.
Various methods have been proposed as wire bonding methods for use in the wire bonding process. The most commonly used method is illustrated in FIG. 3.
In this method, as shown in FIG. 3a, a ball 3a is formed by the spark discharged by an electric torch 5 on a wire 3 which extends from the lower end of a capillary 4. The electric torch 5 is moved in the direction indicated by the arrow. Then, as shown in FIG. 3b, the capillary 4 is moved to a point above the first bonding point 1a, and as shown in FIG. 3c the capillary 4 is lowered so that the ball 3a at the tip of the wire 3 is pressed against the first bonding point 1a. Thereafter, an ultrasonic vibration is applied to the capillary 4 by the horn that holds the capillary 4, thus bonding the ball 3a.
When the ball 3a has thus been bonded to the first bonding point 1a, as shown in FIG. 3d, the capillary 4 is raised and as shown in FIG. 3e moved in the looping direction A so that the capillary 4 is located above the second bonding point 2a. Next, as shown in FIG. 3f, the capillary 4 is lowered so that the wire 3 is pressed against the second bonding point 2a, and an ultrasonic vibration is applied to the capillary 4 by the horn so that the wire 3 is bonded.
Afterward, the capillary 4 is raised to a fixed position; then, the clamper 6 is closed and the capillary 4 and clamper 6 are both raised so that the wire 3 is cut as shown in FIG. 3g.
One wire connecting step is thus completed.
Japanese Patent Application Laid-Open (Kokai) No. 57-87143 and Japanese Patent Application Publication (Kokoku) No. 1-26531 may be cited as examples of this type of prior art wire bonding method.
In the wire bonding method of the prior art as described above, the wire 3 is pushed toward the first bonding point 1a when the application of the ultrasonic vibration in the bonding process of the wire 3 is made to the second bonding point 2a. As a result, bending of the wire occurs, and the "looping linearity", which refers to the shape of the wire extending from a first bonding point to a second bonding point as seen from above, deteriorates. Furthermore, if the width of the second bonding point 2a, which is generally a lead, is narrow or if the surface conditions of the lead are poor, in other words, if the lead is contaminated or oxidized by a paste, etc., then a mere application of an ultrasonic vibration results in poor bonding characteristics (adhesion), and a stable bonding performance is not obtained.